Many applications employing optical devices are limited by optical aberrations that result from the configuration of the optical device. The term “optical aberrations” includes the deviations from the correct function of the optical device that result from disruptions and distortions of the optical path associated with the device. Three optical devices of interest in connection with the present invention are wide-field microscopes, optical tweezers and optical media devices, such as DVD writers and players.
Wide-field microscopes are those which acquire an image of an entire field of view simultaneously, and image both the in-focus and the out-of-focus parts of a sample at once. This is in contrast to confocal microscopes, which are designed to image only one point of a sample at a time. Wide-field microscopy is particularly important in the biological field, where fluorescence is commonly used to image biological samples. In contrast to other types of microscopy, such as confocal microscopy, wide-field microscopy is the optimal methodology for harvesting the most photons in a given optical configuration. When image acquisition is followed by reconstruction methods, such as deconvolution, the out-of-focus contributions are shifted back to the focus, providing enhanced contrast and resolution of three-dimensionally acquired images.
One source of optical aberrations that is particularly limiting in the use of optical devices is the object of the optical device. The object, or sample, refers to the element that the optical device is designed to perform a particular function on. In wide-field microscopes the object is the sample being imaged. In optical media devices the object is the disk which is being written to or read from. Using wide-field microscopy as an example, the acquisition of a high-resolution, three-dimensional image of a cell is severely limited by optical aberrations resulting from the cell itself. Object-induced, or sample-induced, aberrations result from the refractive properties of the object and can be characterized into two groups: spherical aberrations and space-variant aberrations.
Spherical aberrations are mainly contributed by the depth that is being imaged within the sample with refractive index different than the immersion medium. When rays of light travel through a sample of uniform refractive index, they will travel at a certain speed and angle. When those rays of light exit the sample, the rays of light will bend to a different angle and travel at a different speed, according to the refractive index of the environment around the sample, thereby accumulating a different optical path (which is the product of physical path and the refractive index, which determines the phase of the wavefront). The bending of the ray and the changes in optical path causes the light to appear as if it originated from a different point in the sample, and distorts the constructive interference of these rays in the otherwise diffraction-limited image. The change in speed causes the wave-front to become aberrated, since two rays originating from the same point but traveling at different angles through the sample will travel a different distance within the sample and, therefore, experience a phase shift relative to each other. This phase shift could lead to constructive or destructive interference. This causes aberrations to be introduced at the image plane. Typically, an optical device will be designed so that a corrected image can be provided for one particular sample configuration, typically the most common configuration. Since the image is formed by interference of rays emanating from a point at all angles, and brought by well corrected optics to a diffraction limited pattern (that is, all rays reach the geometrical focus with the same phase), the image formed from a point inside a sample with refractive index different than the one for which the optics is corrected (i.e., a different sample configuration than the optical device was designed for) will not result from constructive interference of all rays coming from the source, and will instead become aberrated, Since depth effects are spherical, i.e., they are axially symmetric, two rays of light originating from the same point in the sample, which are traveling through the sample at the same angle but at a different orientation relative to the optical axis, will experience an identical aberration. Furthermore, two rays of light originating from different points in the same depth of the sample, will experience the identical aberration. Accordingly, the depth-induced spherical aberration is space-invariant for all points imaged at a given focal depth. Spherical aberrations cause a fast broadening of the in-focus point image, a fall in the peak image intensity and asymmetric distortions of the out-of-focus areas. Since the spherical aberration is caused by the rays of light traveling through the sample, it becomes more pronounced the longer the rays travel in the sample. Accordingly, the deeper into the sample the microscope is focused, the more aberrated the acquired image becomes due to spherical aberrations.
Space-variant aberrations occur when imaging a sample that does not have a uniform refractive index. Some segments of the sample have different refractive indexes than the rest of the sample, as in the case with most biological samples (inhomogeneous refractive index). For example, a cell has many different organelles, and each may have a different refractive index than the others. When a ray of light travels through the sample, it may travel through a segment which has a different refractive index than the segment it was previously traveling in. Accordingly, the ray of light will bend as it crosses between the two segment, and it will travel at a different speed in each segment. Each ray of light may travel through numerous different segments of different refractive indexes before it exits the sample. Furthermore, since the sample may not be symmetric, the aberrations will be asymmetric, i.e., two rays of light, originating from the same point in the sample, and which are traveling through the sample at the same angle but at different orientations to the optical axis will experience different refraction and phase shifts. Accordingly, space-variant, inhomogeneous, sample-induced aberrations depend on many different factors, including the depth and position in the sample from where the light is originating.
Different solutions have been suggested for correcting optical aberrations. U.S. Pat. No. 6,658,142, incorporated herein by reference, teaches a rigorous deconvolution scheme to computationally correct for sample-induced aberrations in imaging. According to the '142 patent, a) information about the refractive properties of the sample is acquired, b) a location dependent point spread function, which is an aberrated image of a point source, is derived from computationally tracing multiple rays from the point source through the sample according to the refractive information of the sample, and c) a corrected, aberration-free image is provided as the output from a computationally rigorous algorithm. Although effective, this approach significantly slows down the image acquisition process and poses a serious computational burden for image reconstruction.
Methods for correcting sample-induced aberrations in astronomy and confocal microscopy have been proposed using adaptive optics. The term “adaptive optics” refers to optical elements, such as lenses or mirrors, which can be adjusted and reconfigured to various different shapes or configurations. One such solution was proposed in Booth, M. J., Neil, M. A. A., Juskaitis, R. and Wilson, T. “Adaptive aberration correction in a confocal microscope” PNAS. 99(9):5788-5792 (Apr. 30, 2002), incorporated herein by reference. Booth proposes using adaptive optics systems to correct for aberrations in confocal microscopy. This method uses a wave-front sensor to measure the aberrations in the wave-front, and adjusts an adaptive element in the optical path to correct these aberrations. This method is practical for confocal microscopy, in which each acquired image is created point by point, and it is possible, after only a few iterations, to adjust the adaptive element so that the intensity of the image is maximized and the aberrations are minimized. This method, however, requires multiple scans of the sample (with speed and sample damage consequences), and is not applicable for wide-field microscopy, in which the entire sample is depicted in the image, with each point differently aberrated. Since the image includes both in-focus and out-of-focus areas, adjustment to the adaptive element to maximize intensity cannot guarantee reduction of the aberrations, since the sensors cannot distinguish between the in-focus intensity and the out-of-focus intensity.
Similar to wide-field microscopes, optical tweezers are limited by optical aberrations. Optical tweezers, also known as laser tweezers, use the forces of laser radiation pressure to move and trap small particles at the focus of the laser beam. Optical tweezers are commonly used to measure forces on the order of 1-100 piconewtons. They function by producing light intensity gradients which drag particles into the focus of the laser beam, where the radiation forces at all directions are balanced, and the particle becomes trapped. Optical tweezers lose their holding power when the light gradients become weaker and the laser focus becomes blurred. This occurs when optical aberrations are introduced into the system. The deeper into the system the laser focuses, the spherical aberrations become more pronounced and the optical tweezers become less precise. For example, if one attempted to move and trap a polysterene bead deep within a biological system, it may not be possible to trap the bead due to aberrations. To overcome the effect of the aberrations it is possible to increase the laser power, however, at high laser powers it is probable that some damage will occur within the biological system.
Similar to wide-field microscope and optical tweezers, reading and writing information on a media disk, such as a DVD, using optical media devices is limited by optical aberrations. There are currently DVD drives which can write and read data from two layers of information on the DVD. The highest layer is closer to the surface of the DVD, and the second layer is beneath it, further into the DVD. It would be advantageous to be able to read and write data onto as many layers of the DVD as possible, so that more information can be stored on each DVD. In order to read or write data on a DVD, the light beam used to focus on the data needs to stay small enough so that it only focuses on one data bit at a time. However, as a DVD drive attempts to write or read data deep into the DVD, spherical aberrations are introduced. In order to properly write or read a DVD, the light beam used to read or write the data on the DVD needs to focus to a spot small enough to write or read each individual data bit. Furthermore, when using a multilayer DVD, the light beam needs to remain the same size when focused on each layer of data. If the light beam becomes too wide, it may focus on more than 1 data bit at a time, and therefore the data may be miswritten or misread. Current DVD players can not focus on more than two layers of data in a DVD. As the DVD player attempts to focus on more layers, the light beam becomes aberrated, and it widens to a spot that is larger than 1 data bit. Accordingly, any attempt to write or read data in these layers would cause the data to be miswritten or misread. Since these aberrations result from focusing beneath the surface of the DVD, the deeper into the DVD the DVD drive attempts to write or read, the more pronounced the spherical aberrations become.
Some solutions have been proposed to allow for multilayer reading and writing in optical media devices. U.S. Pat. No. 6,974,939, incorporated herein by reference, proposes using two or more different lenses in an optical media device to reduce spherical aberrations. These lenses are designed so as to provide the minimum spherical aberrations possible for reading or writing on the lowest layer on the media disk. Accordingly, the light beam produces a small spot when focused at that layer. When reading or writing on other layers in the media disk, the distance between the two or more lenses is changed to try to reduce the diameter of the light beam when focused at the other layer. However, the adjustment of multi-lens optics is slow and cannot reach optimal aberration correction with a small number of adjustable solid lenses. Therefore, this does not provide a practical solution for writing or reading a DVD with many different data layers.
There still is a need for a method and system for correcting optical aberrations in optical devices. In particular, there is a need for a method and system for correcting optical aberrations in wide-field microscopy, optical tweezers and in optical media devices which overcomes the disadvantages of the prior art approaches.